1. Field of the Invention
The present invention relates to a water-soluble heat-press-bonding polyvinyl alcohol type (hereinafter referred to simply as PVA type) binder fiber. More particularly, the present invention relates to a PVA type binder fiber which is heat-press-bondable, small in dimensional change of fiber during heat-press bonding, and water-soluble even after heat-press bonding; a process for production of said fiber; and a nonwoven fabric using said fiber.
2. Description of the Prior Art
Heat-bonding binder fibers made from, for example, a melt-spinnable polyethylene or polyester are on the market. Recently, a sheath-core bicomponent type heat-bonding binder fiber comprising a high-melting-point (hereinafter referred to simply as high-melting) polymer as the core and a low-melting-point (hereinafter referred to simply as low-melting) polymer as the sheath has been developed, and this has made it possible to suppress the shrinkage of fiber during heat bonding. The sheath-core bicomponent type heat-bonding binder fiber is finding wider applications owing to its merits such as easy and speedy bonding operation, no public hazard and the like.
These heat-bonding binder fibers, however, are each made from a hydrophobic resin and therefore have low bondability to hydrophilic resins such as PVA type resin, cellulose type resin and the like. Further, these heat-bonding binder fibers are not water-soluble, of course.
In producing a water-soluble nonwoven fabric, there has been used a process which comprises imparting an aqueous solution of a water-soluble resin of PVA type to a web of a water-soluble fiber of PVA type and then drying the resulting web at low temperatures for a long time to give rise to fixing between fibers. For example, in producing a chemical lace base fabric which must be water-soluble, there is generally used a process which comprises coating or impregnating a dry laid nonwoven fabric made from a water-soluble PVA fiber, with an aqueous solution of a PVA type resin and then drying the resulting fabric. In such a process of imparting an aqueous solution and then drying the resulting material, however, the water-soluble fibers of the base fabric cause swelling because of the imparting of an aqueous solution thereto and, when the drying temperature is high, dissolve in the aqueous solution, which causes the deformation of nonwoven fabric; therefore, the drying must be conducted at low temperatures, which requires a long drying time and results in low productivity. Incidentally, the above-mentioned "chemical lace base fabric" is a water-soluble fabric or nonwoven fabric used as a base for production of lace. When mechanical embroidery is made on the base fabric with a water-insoluble thread and then the base fabric is dissolved and removed by an aqueous treatment, the embroidery remains in the form of lace.
Development of a heat-bonding water-soluble fiber allows for fixing between fibers by heat bonding and enables high productivity. In producing a base fabric for wet wiper, for example, by bonding the fibers of a cellulose base material by the use of a heat-bonding polyolefin type fiber, the product of inferior quality or the refuses from trimming all appearing during the production of said base fabric are not recoverable and therefore are disposed by incineration; in this case, if the heat-bonding fiber is water-soluble, the product of inferior quality or the refuses from trimming are recoverable because the bonded fibers can be disintegrated simply by washing with water.
All of conventionally known heat-bonding fibers are produced from a melt-spinnable hydrophobic polymer, and no fiber is known yet which has both water solubility and heat bondability and yet has fiber properties capable of withstanding the conditions of actual use. For example, a PVA type polymer, which is a typical water-soluble polymer, has a strong interaction between molecules owing to the hydroxyl groups in the molecule, has a melting point close to the thermal decomposition temperature, and is generally impossible to melt without causing thermal decomposition; therefore, it is generally impossible to produce a heat-bonding fiber from said PVA polymer.
Under such a circumstance, it was proposed to allow a PVA type polymer to have a lower melting point or a lower softening point for enabling its melt molding or for using it as a hot-melt adhesive, by applying, to the PVA type polymer, a means such as internal plasticizatin (by copolymerization modification or post-reaction modification) or external plasticization (by plasticizer addition). Water-soluble hot-melt PVA type adhesives are disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 87542/1976, U.S. Pat. No. 4,140,668 and Japanese Patent Application Kokai (Laid-Open) No. 50239/1978. Each of these hot-melt PVA type polymers, however, has a low polymerization degree of 600 or less so as to be able to give a melt of low viscosity and high adhesivity and therefore has a very low spinnability. Moreover, each of the resulting fibers, when used as a heat-bonding fiber, shows high shrinkage because the oriented molecules in fiber melt and relax during heat bonding; therefore, each fiber is difficult to put into actual use.
In Japanese Patent Publication No. 29579/1972 and Japanese Patent Publication No. 42050/1972, it is described that a fiber obtained by wet spinning of a mixture of a PVA solution with an ethylene-vinyl acetate copolymer emulsion is heat-sealable and can be used as a binder fiber or base fiber for paper or nonwoven fabric. In this technique, however, said emulsion to be mixed with a PVA solution must be an emulsion of a water-in-soluble polymer. Since a water-soluble polymer cannot be made into an emulsion, the above technique is unable to produce a water-soluble fiber.
In Japanese Patent Publication No. 6605/1966 and Japanese Patent Publication No. 31376/1972, it is described that an easily fibrillatable fiber is produced by mix-spinning a completely saponified PVA having a saponification degree of 99.5 mole % or more and a partially saponified PVA. In these prior arts, it is intended to produce an easily fibrillatable fiber; therefore, a highly water-resistant completely saponified PVA is used as one component, there are carried out drawing, heat shrinkage and, an necessary, acetalization and, as a result, the resulting fiber is not water-soluble. Further, in these prior arts, there is used a dehydration coagulation method employing an aqueous Glauber's salt solution as a coagulation bath, which is an ordinary spinning method used for vinylon; in this dehydration coagulation method, however, there is formed a fiber of nonuniform cross section having an obvious skin-core structure. Moreover in the dehydration coagulation method, it is difficult to spin a partially saponified PVA having a saponification degree of 85 mole % or less and, when the resulting fiber is subjected to washing with water in order to remove the Glauber's salt adhereing onto the fiber surface, the fiber surface dissolves in the water used for washing and there occurs fusion between filaments. For this reason, it is actually impossible in the prior arts to use a partially saponified PVA having a saponification degree of 85% or less and conduct mix-spinning. In fact, all Examples use, as the partially saponified PVA, PVAs having a saponification degree of 88 mole % or more.
In Japanese Patent Publication No. 28729/1976, it is described that a self-adhering synthetic pulp is produced by dissolving a PVA, a polyacrylonitrile and an acrylonitrile-grafted PVA in dimethyl sulfoxide (hereinafter referred to simply as DMSO) (DMSO is a common solvent for said three polymers), subjecting the solution to wet spinning, drawing the resulting fiber, and subjecting the drawn fiber to beating. In such a technique, however, no water-soluble fiber is obtainable, of course.
In Japanese Patent Application Kokai (Laid-Open) No. 5318/1977, it was proposed to produce an ultra-fine fiber by mix- or bicomponent-spinning a PVA of low polymerization degree and low saponification degree and a polymer having a fiber formability and then washing the resulting filaments with water to remove the PVA of low polymerization degree and low saponification degree. Since the polymer having a fiber formability is a water-insoluble polymer not affected by the above water treatment, no water-soluble fiber is obtainable by the above technique.
In Japanese Patent Application Kokai (Laid-Open) No. 260017/1989, there was proposed a high-strength water-disintegratable PVA type bicomponent fiber comprising, as the core component, a PVA type polymer having a saponification degree of 80-95 mole % and, as the sheath component, a PVA type polymer having a saponification degree of 96 mole % or more. This bicomponent fiber, unlike the binder fiber of the present invention, basically has a core-sheath structure in which the core is present as one core and the surface layer consists of a thick layer of a high-melting polymer, and therefore is unusable as a heat-bonding fiber.
In European Patent No. 351046, there is described a process for producing a highly-water-resistant high-shrinkage PVA type fiber by mix-spinning a PVA and a polymer capable of crosslinking with the PVA (e.g. a polyacrylic acid) and then subjecting the resulting fiber to a crosslinking reaction. The fiber obtained by this process causes breaking in water of 100.degree. C. or less because the uncrosslinked portions of the fiber dissolve in the water. However, the crosslinked portions of the fiber are insoluble in the water.
It is strongly desired in the art to develop a PVA type binder fiber which has both heat bondability and water-solubility and which has fiber properties capable of withstanding the conditions of actual use. Such a binder fiber, however, has been unobtainable with conventional techniques.